From: Ronald Wong (ronwong@inreach.com)
Date: Mon Aug 18 2003 - 13:00:42 PDT
Message-Id: <l03102801bb64381f1805@[209.209.19.156]> Date: Mon, 18 Aug 2003 13:00:42 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: re: an old black and white tv
Malcolm asked:
>I have two questions in regards to a (black and white) tv. First, is it =
>true that the static we obsereve comes from the big bang? Second, when I=
> put a magnet to the screen does it bend toward the positive pole and awa=
>y from the negative? Istn't there a beam of electrons being shot at the =
>screen?
Let's start with "First".
The answer is yes. Some of the snow that you see on the screen comes from
the cosmic microwave background (CMB) - but not much.
As Al Sefl said:
>The "snow," random radio white noise, you see on a television is mostly from
>background radio waves generated naturally from many "local" sources. The Sun
>and Jupiter are the greatest sources of radio energy in the solar system.
>Electrical discharges of lightening on Earth is another major contributor.
>Power lines and other man made sources also show up.
Apparently, the universe has been expanding ever since the "Big Bang"
occurred. Over the billions of years of expansion, it has cooled down to
the current temperature of around 3 K (Kelvin). The cooling down is due to
the increase in the wavelengths of the radiant energy that was present when
the expansion began. The increase in wavelengths is due to the Doppler
effect associated with the expansion. The CMB simply reflects the current
state of this radiant energy. With that in mind, it should be clear that we
are dealing with a thermodynamic phenomena. This means that instead of a
single frequency/wavelength of radiant energy associated with the CMB there
is a range of frequencies/wavelengths.
The distribution of these wavelengths does not follow the popularly known
bell-shaped curve of Maxwell. The actual curve is well known to physics
teachers but, in all likelihood, not so well known to teachers of physics.
It's referred to as the "Planck distribution" curve. If you plot the
energy/vol/wavelength-range (or the relative intensity) against wavelength,
you get a curve that rises very rapidly at first to a maximum value and
then gradually falls off as the wavelengths get longer (sort of like an
elevation profile of the Sierra Nevada in California where the elevation
increases very rapidly as you travel the short distance from Mono Lake to
Tioga Pass and then tapers off as you continue west towards the San Joaquin
Valley).
All "black bodies" at a temperature greater than 0 K give off a range of
radiant energy (EM waves) which fits this distribution curve. At high
temperatures, like that at the surface of the sun, the peak is in the
yellow-red portion of the visible spectrum. The curve then tapers off into
the red and far-infrared regions of the EM spectrum. For bodies of
correspondingly less and less temperature, the peak shifts towards longer
and longer wavelengths (thus the red of the old, cooler stars in our
universe). At 2.7 K it peaks in the microwave portion of the EM spectrum at
a wavelength of about 1 mm and then tapers off into the radio/tv region of
the spectrum.
Penzias and Wilson had a detector that they were planning to use to study
galactic radiation. They needed to purge the detector of any sources of
random EM radiation and decided to use an EM wave with a wavelength of
7.35 cm to achieve this objective. This happens to be in the microwave
region of the EM spectrum. What they discovered was that a mysterious
radiation of just this wavelength was coming from everywhere in the
universe (i.e. cosmic) and, in terms of the "antenna temperature",
corresponded to about 3 K. Voila! CMB.
Over the years, almost two dozen similar investigations have been made at
other wavelengths and, within experimental error, all the results -
including that of Penzias and Wilson - were found to lie along the Planck
distribution for the black body radiation of an object (which in this case
is the universe) whose temperature is 2.7 K.
As you can see from the above, the thermal properties of the CMB causes a
little bit of the longer wavelengths associated with it to overlap the
TV/FM/radio wavelengths. As a result, some of the snow that you see on your
TV screen when it is tuned to an unused channel is due to the CMB.
The contribution is very small. It amounts to only about 1 % of the snow
you see on the screen.
Al went on to say that:
>A television sensitivity in measured in many microVolts...
to suggest that the amplitude of the CMB would be insufficient to produce
snow on the screen.
A receiver's sensitivity has to do with the signal to noise ratio. A signal
whose strength is above the prescribed voltage is intelligible. Below that
level it produces noise (i.e. static or, in the case of TV, snow).
We are not dealing with a signal. CMB is nothing but noise (it's a
thermodynamic phenomena) and, like the signals from the sun and Jupiter, it
can contribute to the snow on the TV screen even though it, like the
others, doesn't generate voltages in the antenna above the sensitivity
specified for a TV receiver. Compared to the other sources, it just doesn't
contribute much because very little of the CMB is in this part of the EM
spectrum.
***********************************
Now, on to "Second":
>when I=
> put a magnet to the screen does it bend toward the positive pole and awa=
>y from the negative? Istn't there a beam of electrons being shot at the =
>screen?
The answer to the second question is yes.
The answer to the first question is no.
What bends when you place a magnet near the screen is the electron as it
moves through the magnetic field of the magnet.
It bends neither towards nor away from a magnet's N(orth)/S(outh) pole (NOT
+/- pole by the way).
It bends in a direction that is perpendicular to BOTH
a. the direction that it is moving and
b. the magnetic line of force that runs from one magnetic pole to the other.
Let's say you have a horseshoe magnet and place it on the center of a TV
picture tube so that the N-pole of the horseshoe magnet is on your left and
the S-pole is on your right. By convention, the magnetic lines of force run
from the N-pole to the S-pole.
The electron is coming directly towards you as it moves from the back of
the picture tube, where the source of electrons are, to the screen.
Because it is crossing the magnetic lines of force (i.e. it isn't moving
parallel to them), there will be a force acting on it deflecting it from
it's original path. The direction that it will be deflected can be
determined using the LEFT hand rule (LEFT-hand because we are dealing with
a NEGAtively charged particle).
There are a number of versions of the "hand rules". The one that I found
that worked the best with my students involves opening up your hand so that
the thumb and fingers all lie in one plane with the thumb perpendicular to
the extended fingers.
Whether the charges were positive (right hand) or negative (left hand), the
rule was the same: position your hand so that the thumb pointed in the
direction of motion of the charges and the fingers pointed in the direction
of the magnetic lines of force. The force acting on the moving charge
extends perpendicularly from the palm of the hand ("away from the palm").
When I first demonstrated the rule to the students, I oriented my hand
appropriately with the charge's direction of motion and the magnetic field
and then moved my hand in the direction of the force so that the students
could see the hand/force "pushing" the charge in the prescribed direction.
Because the hand motion is one that they are all familiar with (who hasn't
pushed something with their hands?), they quickly learned how to implement
this hand-rule correctly.
In your case, you can use the hand rule to predict which way the electron
(and the image around the magnet in general) will be deflected when it
moves towards you between the two pole faces (answer: downward - unless
someone has reversed the polarity of your magnets). You can then verify it
by simply placing the properly oriented magnet in the center of the TV's
screen. You can then have the kids predict what will happen when you place
magnet on the screen with the N-pole below the S-pole and then verify their
predictions. Ask them to try to explain (hypothesis time!) why the picture
around the magnet gets distorted the way it does and then see if they can
find a way to test their explanation - and then perform the test.
I would suggest that you do all of this using an old discarded-but-useable
TV set (or one that comes with a degausser).
Have fun.
ron
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